This invention relates generally to ion and plasma technology, and more particularly it pertains to plasma and ion sources with closed electron drift.
This invention can be used in industrial applications such as sputter etching, sputter deposition, and property enhancement. It can also find application in electric space propulsion.
The acceleration of ions to form energetic beams of such ions has been accomplished both electrostatically and electromagnetically. The present invention pertains to sources that utilize electromagnetic acceleration. Such sources have in general been called electromagnetic or gridless ion sources. Because the ion beams are typically dense enough to require the presence of electrons to avoid the disruptive mutual repulsion of the positively charged ions, the ion beams are also neutralized plasmas and these ion sources are also called plasma sources. When the ion sources are used for space propulsion, they are called thrusters.
In ion sources (or thrusters) with electromagnetic acceleration, there is a discharge between an electron-emitting cathode and an anode. An electric field for accelerating ions is established by the interaction of the electron current in this discharge with a magnetic field created in the discharge region between the anode and cathode. This interaction generally includes a Hall current normal to both the magnetic field direction and the direction of the electric field that is established. This Hall current consists primarily of electrons.
The present invention pertains to a Hall current ion source, i.e., one that employs a Hall current, where the discharge region has a generally annular shape with both inner and outer boundaries, and where the ions are accelerated only over the annular cross section of this region. This type of Hall current ion source is also called a closed-drift source because the Hall current of drifting electrons follows a closed path around the annular discharge region. This type of Hall-current ion source usually has a generally radial magnetic field shape in the discharge region as described in U.S. Pat. No. 5,359,258xe2x80x94Arkhipov, et al., U.S. Pat. No. 5,763,989xe2x80x94Kaufman, and a review paper by Zhurin, et al., in Plasma Sources Science and Technology, Vol. 8, beginning on page R1. These publications are incorporated herein by reference.
It should be noted that a Hall-current ion source can also have a circular discharge region with only an outside boundary, where the ions are accelerated continuously over the circular cross section of this region. This type of ion source is called an end-Hall ion source and has a generally axial magnetic field shape as described in U.S. Pat. No. 4,862,032xe2x80x94Kaufman et al, and an article by Kaufman, et al., in Journal of Vacuum Science and Technology A, Vol. 5, No. 4, beginning on page 2081. These publications are incorporated herein by reference. This type of ion source is mentioned to distinguish it from the closed-drift ion source of interest herein.
It should be further noted that the closed-drift ion source of interest herein is generally of the magnetic-layer or SPT (stationary plasma thruster) type. The differences between this type of closed-drift ion source and the other major closed-drift type, the anode-layer type, are described by Zhurin, et al., in the aforesaid review paper in Plasma Sources Science and Technology, Vol. 8, beginning on page R1. In geometry, the magnetic-layer or SPT type has a discharge region that has a length greater than its width, while the anode-layer type has a discharge region that has a length less than its width, where the width of the discharge region in both cases is the radial distance between the inner and outer boundaries of the discharge region. The preferred magnetic field configuration for the magnetic-layer type is one where the magnetic field is generally radial, concentrated near the exit plane, and has a much decreased strength near the anode at the upstream end of the discharge region.
There is interest in small ion sources, as indicated by Guerrini, et al, in Proceedings of the 24th International Electric Propulsion Conference (Moscow, 1995) beginning on page 259; by Guerrini, et al, in Proceedings of the 25th International Electric Propulsion Conference (Cleveland, Ohio, 1997) beginning on page 326; and by Khayms, et al., also in Proceedings of the 25th International Electric Propulsion Conference (Cleveland, Ohio, 1993) beginning on page 483. These publications were directed primarily toward electric space propulsion, but there is also interest in small ion sources for industrial applications as indicated by the commercial Mark I end-Hall ion source manufactured originally by Commonwealth Scientific Corporation and now manufactured by Veeco Instruments Inc.
One might expect that a small closed-drift ion source could be made by geometrically scaling down a larger source of the same typexe2x80x94i.e., by reducing the dimensions of all parts by the same factor. The flux densities in the permeable portions of the magnetic circuit will increase if this approach is carried out, and will reach a saturation value in some part of the magnetic circuit if there is sufficient reduction in size. Because space is most limited in the region within the inside diameter of the discharge region, the saturation value will usually be reached in the inner path of the magnetic circuit, typically at the upstream end of this element of the magnetic circuit
In light of the foregoing, it is an overall general object of the invention to provide a magnetic field configuration suitable for a small closed-drift ion source that performs efficiently over a wide operating range, is generally of the magnetic-layer or SPT type, and can be used in a variety of industrial and space propulsion applications that require an ion source or thruster of small size.
Another overall general object of the invention is to provide a magnetic field configuration that is efficient in the use of magnetic circuit elements so that it is suitable for a larger closed-drift ion source that is of the magnetic-layer or SPT type and is compact, efficient, and economical in the use of magnetically permeable material for the ion beam energy and current generated.
A specific object of the present invention is to optimize the shape of the magnetic field without the use of an inner electromagnet which would reduce the permissible cross section of the inner path of the magnetic circuit and add resistive heating to one of the hottest regions of a closed-drift ion source.
Another specific object of the present invention is to minimize the magnetic flux passing through the inner path of the magnetic circuit that does not directly contribute to the ionization and acceleration process, thereby reducing the flux density in that element of the magnetic circuit.
A more general object of the present invention is to minimize the gas flow required for operation by making a closed-drift ion source that has a discharge region with a small mean diameter.
In accordance with one specific embodiment of the present invention, a compact closed-drift ion source takes a form that includes a means for introducing a gas, ionizable to produce a plasma, into an annular discharge region. An anode is located at one end of this region and an electron-emitting cathode is located near the opposite and open end. A magnetic circuit including magnetically permeable elements and at least one magnetizing means extends from an inner pole piece to an outer pole piece, with both pole pieces located near the open end. The electron current from the cathode to the anode in the discharge region interacts with the magnetic field therein, ionizes the gas to generate ions, and accelerates these ions out of the open end. Permeable elements of the magnetic circuit form a permeable enclosure that surrounds the anode end of the discharge region. Adjacent elements of the permeable enclosure, the inner pole piece, and any intermediate permeable elements are in close proximity, one to the next. A magnetizing means is located only between the outer pole piece and the permeable enclosure.